I don't think this tells us much. The present distribution of supporters is rather unique in how strongly it correlates with population density, which means that Dems are going to have a major structural handicap in Senate. I don't think there was ever anything similar historically. And it won't change unless and until the coalitions change, which, sure, will happen eventually - but then it'll be a completely different party under the same brand, so why would Republicans today care about that new party's difficulties?